FIG. 1 shows a typical backside illuminated detector element 1 comprising a substrate 10 having a back surface 12 and a front surface 14. A layer of detector material 16 is bonded to front surface 14 to form the detector element 1. Detector layer 16 comprises layers of opposite type P and N layers whose interface forms a PN semiconductor junction 15. Semiconductor junction 15 operates as a photovoltaic detector, which produces-for example-an open circuit voltage when the junction interacts with a photon.
Incident light strikes back surface 12 of substrate 10, passes through substrate 10 and is then absorbed by detector material 16. Substrate 10 is formed from a material substantially transparent to the optical wavelengths of interest - for example, a wavelength of 8-12 microns for infrared (IR) sensing. Substrate 10 can be a cadmium telluride (CdTe) crystal while detector material 16 can be a mercury cadmium telluride (HgCdTe) layer.
If substrate 10 and detector layer 16 are processed to form an array of detector elements 1, conventional array addressing techniques can address any individual detector element.
Laser beams can damage optical sensor systems-particularly sensor arrays. The system (not shown) increases the intensity of incident light by several orders of magnitude before the light reaches the system's focal plane. Detector layer 16 is positioned in the focal plane of the sensor system, the point of maximum optical intensity. Unlike substrate 10 which transmits the light, detector material 16 absorbs the light's energy. Thus, detector element 1, the "eye" of the system, is the system's most easily damaged component.